Cathodic sputtering is widely used for the deposition of thin layers of materials onto desired substrates. Basically, this process requires gas ion bombardment of a target having a sputtering surface formed of a material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a heat conducting backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target raceway.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in positions such that the above-noted magnetic field forms the shape of a loop or tunnel extending around the exposed face of the target.
There has been a growing interest in sputtering target assemblies with increasingly different thermal expansion coefficients between the target material and the backing plate material. Although sputtering target assemblies may be made by solder bonding backing plates of various materials to a target, solder bonding has the disadvantage of not being able to withstand high power sputtering applications. Thus, diffusion bonded sputtering target assemblies are preferred.
Diffusion bonds are produced by pressing the surfaces into contact while applying heat to induce metallurgical joining and diffusion to varying extent across the bond interface. A variety of different metal combinations may be used as bonding aids. These metals are applied as coatings to one or more of the DB interfacial surfaces to promote DB bonding and may be applied via conventional electroplating, electroless plating, vacuum cadmium plating, physical vapor deposition, or other techniques. In some cases, a metal foil is positioned between the surfaces to be joined by DB. Commonly, the surfaces to be joined are prepared by chemical or other means to remove oxides or their chemical films which may interface with the bonding process.
Diffusion bonding techniques include hot isostatic pressing (HIP) and uniaxial hot pressing (UHP). In UHPing, the target and backing plate are placed between a pair of platens, or the like, in a chamber which provides for careful control of temperature, pressure and other atmospheric conditions. The controlled atmosphere may be that of a vacuum, inert, or reducing gas. The assembly is heated to a temperature that is below that of the lower melting member of the target/backing plate combination. As the assembly is heated, pressure is applied by the platens acting in a uniaxial direction. The assembly is maintained in the control chamber until a strong DB bond is formed.
In the HIPing process, the target/backing plate assembly is placed within a canister. A vacuum is drawn on the canister and then it is placed in a HIPing chamber. An argon or helium atmosphere is charged into the canister and temperature and pressure are increased. The HIPing canister is subjected to pressure from all sides. As in the UHPing process, the temperature approaches the melting joint of the lower melting member of the target/backing plate combination. Then, the HIPed assembly is maintained at the desired temperature, pressure, and atmospheric conditions to form a strong DB bond.
After bonding has occurred, the assembly is cooled. The cooled target usually bows or deflects to one direction. This deflection is usually caused by unequal shrinkage or expansion of the target and backing plate due to the differing CTE of each. Cracking of the bond between the target and backing plate, or delamination may also occur during cooling. In addition, the different CTE materials render the assembly susceptible to other stresses, degradations or distortions during the high temperatures that occur during sputtering. To remove the deflection and create a “flat” assembly target surface, the target is typically flattened mechanically.
This diffusion bonding and subsequent flattening process is suitable for ductile targets such as titanium or tantalum. For brittle targets such as tungsten, however, the likelihood of delamination is greater throughout the process. The bond strengths between the layers of such target assemblies are typically less than 45 MPa.